Coating materials, e.g. polymers, are often used as protective barriers on circuit boards, electrical components, medical devices and the like. Parylene is a generic term often used to describe a class of poly-p-xylylenes which are derived from a dimer of the structure: ##STR1## where X is typically hydrogen or a halogen. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, parylene is ideally suited for use as a conformal coating.
Typically, parylene is applied by vapor deposition under vacuum conditions wherein the parylene monomer is condensed and Polymerized directly on the article to be coated. Since the parylene monomer is not stable, the parylene dimer, as illustrated above, is used as the starting material.
In most applications, parylene is deposited on articles by a pyrolytic deposition process which begins with the vaporization of the parylene dimer. The dimer is pyrolytically cleaved at temperatures of about 500.degree. to 750.degree. C. to form a reactive parylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the articles are located. Within the deposition chamber, the reactive monomer vapor condenses and polymerizes upon the articles to form a coating of parylene on the article. Any monomer vapor which fails to condense within the deposition chamber is typically removed by a cold trap which is maintained at cryogenic temperatures.
The process is generally carried out as a batch process in a closed apparatus under vacuum conditions, typically from about 10 to 50 millitorrs. Such an apparatus typically comprises separate zones for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such zones being connected by way of appropriate plumbing or tubular connections.
When it is desired to coat multiple articles, a tumble coating apparatus is often employed. In a tumble coating apparatus, the deposition chamber comprises a fixture for containing the articles, e.g., a drum, which is oriented horizontally and rotatably mounted within a housing (sometimes also referred to as a vacuum chamber). The articles to be coated are tumbled in the fixture during the coating process. Typically, small articles, such as ferrite cores used in the pulse transformer industry (known in the art as toroids), ferrite and permalloy bobbins used in the coil windings industry, headpole pieces used in the disc memory industry, small rotor and stator assemblies used in the automotive, aerospace and computer industries, mint coins, jewelry and the like, are coated in this manner.
Examples of common tumble coating apparatus are disclosed in U.S. Pat. No. 4,758,288 issued Jul. 19, 1988. In such apparatus, the fixtures resemble drums which have a solid wall construction, hereinafter referred to as a "drum system". The vaporized parylene monomer is typically introduced by a feed tube which extends into the rotating drum. Since the process is conducted under vacuum conditions, an open annulus is typically provided around the feed tube in order to allow communication between the inside of the drum and the housing where the vacuum is drawn. As a result, the presence of the open annulus limits the volume of articles that can be placed within the drum. Moreover, despite the presence of the annulus, a pressure gradient can be created between the drum and housing thereby restricting the gas flow within the drum. Such a Pressure gradient can result in an uneven coating thickness. Also, with such drum systems, it is often undesirable to increase the volume to accomodate more articles because the parylene coatings, and even the articles themselves, can be subjected to mechanical damage caused by the large volume of articles tumbling upon themselves.
Occasionally, it is desirable to coat several types of articles or different lots of the same article at one time. However, this cannot be conveniently done with a typical drum system, since the articles would be mixed during the tumbling.
In view of the limited capacity of typical drum systems, coating apparatus are desired which can provide increased capacity without causing mechanical damage to the articles being coated. In addition, improved fixtures having separate cells are desired to replace the drums so that different articles can be coated without intermixing. Furthermore, improved coating apparatus are desired which can provide a relatively constant pressure throughout the deposition chamber.